1. Field of the Invention
This invention relates generally to a system and method for controlling relative humidity (RH) cycling in a fuel cell stack and, more particularly, to a system and method for controlling RH cycling during low power transients of a fuel cell stack that includes damping the power request, and using the extra stack power to charge a battery.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The dynamic power of a fuel cell system is limited. Further, the time delay from system start-up to driveability and low acceleration of the vehicle may not be acceptable. During a drive cycle, the stack cell voltage varies because the variable driver power request follows the stack polarization curve. The voltage cycles can decrease the stack durability. These drawbacks can be minimized by using a high voltage battery in parallel with the fuel cell stack. Algorithms are employed to provide the distribution of power from the battery and the fuel cell stack to meet the requested power.
For the reasons discussed above, some fuel cell vehicles are hybrid vehicles that employ a rechargeable supplemental power source in addition to the fuel cell stack, such as a DC battery or a super capacitor (also referred to as an ultra-capacitor or double layer capacitor). The power source provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. More particularly, the fuel cell stack provides power to a traction motor and other vehicle systems through a DC voltage bus line for vehicle operation. The battery provides the supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW or more of power. The fuel cell stack is used to recharge the battery at those times when the fuel cell stack is able to meet the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the battery through the DC bus line.
FIG. 1 is a schematic block diagram of a fuel cell system 10 including a fuel cell stack 12 and a battery 14 that includes power electronics. In order to provide battery charge or discharge, a voltage difference is needed between the stack voltage and the battery voltage that is greater than or equal to the battery charge. When the stack voltage is greater than the battery voltage, the power electronics operates as a voltage amplifier where the gain is less than or equal to one. The fuel cell stack 12 provides electrical power to a high voltage bus line, represented here as positive bus line 16 and negative bus line 18. In a vehicle fuel cell system, the fuel cell stack 12 may include about 400 fuel cells. The battery 14 is also coupled to the high voltage bus lines 16 and 18, and provides supplemental power as discussed above. A compressor 30 provides cathode input air to the stack 12 on inlet line 32. A flow meter 34 measures the flow of the cathode input air to the stack 12.
The fuel cell system 10 includes a power inverter module (PIM) 22 electrically coupled to the bus lines 16 and 18 and an AC or DC traction motor 24. The PIM 22 converts the DC voltage on the bus lines to an AC voltage suitable for the AC traction motor 24. The traction motor 24 provides the traction power to operate the vehicle, as is well understood in the art. The traction motor 24 can be any suitable motor for the purposes described herein, such as an AC induction motor, an AC permanent magnet motor and an AC three-phase synchronous machine. During regenerative braking when the traction motor 24 is operating as a generator, electrical AC power from the motor 24 is converted to DC power by the PIM 22, which is then applied to the bus lines 16 and 18 to recharge the battery 14. A blocking diode (not shown) prevents the regenerative electrical energy applied to the bus lines 16 and 18 from flowing into the fuel cell stack 12, which could otherwise damage the stack 12.
For a typical hybrid vehicle strategy, the battery 14 is mainly used to increase efficiency, lower the dynamic requirements of the fuel cell system, and/or increase the performance of the vehicle. If the traction motor 24 demands more power, the battery 14 can provide the stored energy to the motor 24 very fast.
During low power transients when the requested output load on the fuel cell stack goes from a high power output to a low power output, the reactant gas flow for the anode and cathode side of the stack reacts accordingly to reduce the flow. The flow of the hydrogen to the stack can be reduced very quickly, on the order of about 100 milliseconds, in response to the reduced power demand. However, the compressor providing the cathode air is a relatively large machine having a high inertia that takes a few seconds to reduce the cathode reactant air flow. During the low power transients, the amount of product water produced by the fuel cell stack will be significantly reduced as a result of the low flow of hydrogen. However, the continued cathode airflow through the stack as a result of compressor inertia will have a drying effect on the membranes, which produces drying and subsequent RH cycling of the membranes during low power transients. Membrane RH cycling causes the membrane to expand and contract as a result of the absorption of water and subsequent drying. This expansion and contraction in the membrane causes pin holes in the membrane, which create hydrogen and oxygen cross-over through the membrane creating hot spots that further increase the size of the hole in the membrane, thus reducing its life. This RH cycling is a major reason for premature MEA failures in fuel cell stacks. Therefore, it is desirable to reduce RH cycling in the fuel cell stack.